Abstract
A cyclone-tracking algorithm is used to identify monsoon low-pressure systems (LPS) in the ERA-interim re-analysis (1979–2010). The LPS that are connected to observed extreme rainfall events are picked out and studied with a focus on their dynamic and thermodynamic structure. Cyclone composite clearly shows the general structure of the LPS, with a pronounced cold core at lower levels and warm core aloft. Evaporative cooling from the falling precipitation is proposed to generate the cold core. The temperature gradients across the cyclone centre are strongest in the early phase of the low. We suggest the baroclinic instability to be important in the development phase of the LPS, whereas the upward motion ahead of the low is maintained through latent heat release in the mature phase. This cooperation between the large-scale flow and the cumulus convection is known as the conditional instability of second kind (CISK). From the composites of the time steps where the extreme precipitation is occurring, a colocation of the strong updraft and vertical velocity is shown. Based on this, we suggest the extreme rainfall events to be a result of the LPS dynamics, which is dominated by the CISK mechanism at this stage of the low. Correlation and co-variability between the LPS precipitation and different meteorological parameters are performed, and we find the LPS precipitation to show a large sensitivity to variability in the vertical velocity and specific humidity at 750 hPa.
Highlights
The South Asian monsoon circulation, which is present over India normally from the beginning of June to the end of September, releases up to 70% of the total annual rainfall in India (Webster and Fasullo, 2003; Tyagi et al, 2012)
We investigate the sensitivity of the parameters important for the precipitation associated with the monsoon low-pressure system (LPS) during an extreme rainfall event
For each LPS, the time step when the precipitation exceeds the 99.5th percentile is picked out, and a composite of when there is extreme precipitation (Pmax) is constructed
Summary
The South Asian monsoon circulation, which is present over India normally from the beginning of June to the end of September, releases up to 70% of the total annual rainfall in India (Webster and Fasullo, 2003; Tyagi et al, 2012). Shukla (1978) highlighted the importance of the combination of the barotropicÁbaroclinic instability and the conditional instability of second kind (CISK; Charney and Eliassen, 1964) for the development of the depressions They suggested that the primary driving mechanism for the growth of the monsoon depressions is the CISK theory, which is a cooperate feedback between the low-level convergence of the large-scale flow and the latent heat of condensation. The method is based on a cyclonetracking algorithm (Hodges, 1994, 1995, 1999), and by performing sensitivity tests we discovered that it is not straightforward to develop an objective procedure to identify the systems This is described, together with the method of connecting the LPS to extreme rainfall events. This work will be presented in its own manuscript in the near future
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